This is one of those contructions that if it didn't have the Christian cross I would swear it was a Mosque, notice the 2 round domes, the archs and even the tower of the minaret!!! I don't know when exactly this one was built but it seems to me another Christian Church built on top of an old Mosque as it was popular to do at the time of the reconquest. Not Only Serpa was an Area deeply into Moorsih/Muslim territory but also it was an important centre of commerce of the Alentejo. Nowadays there is a Geodesic mark on top of the minaret people seem to believe that it wasn't a minaret but a watchtower (in portuguese: torre de atalaia) to be honest I've got my doubts...It looks really an old masjid to me...check the Photos